This application claims the benefit of priority German Application No. 199 03 619.5, filed Jan. 29, 1999.
The present invention relates to powder-metallurgically produced composite materials comprising a matrix with a granular additive embedded therein, which consists of at least two refractory components present as mixed crystals or intermetallic phases. Furthermore, the invention relates to a method for their production and their use as contact materials, preferably in electrical vacuum switch boxes.
Vacuum contact pads form the core in switch boxes of electrical vacuum switches and according to the prior art generally comprise an arc-resistant, granular component (refractory metals such as for example W, Mo or Cr) embedded in a metal matrix with a low melting point and a high conductivity (e.g. Ag, Cu, or alloys thereof). Contact materials have to meet high and sometimes contradictory demands, such as
low material consumption,
sufficient breaking capacity,
low welding tendency,
low electrical resistance,
good disruptive strength (electric strength),
low break voltage.
CuCr composite materials have turned out to be especially suitable for vacuum power switches with an average voltage in the range of  greater than 12 kV to about 30 kV and higher. CuCr material have very favorable circuit breaking properties and a good electric strength (dielectric resolidification). The material consumption of CuCr materials is sufficiently low for the low number of 10,000 operations required in this range of performance.
In the low voltage range  less than 1,000 V, the use of vacuum contactors is increasingly gaining importance. The contactor used in this range have to withstand 1,000,000 and more operations and the break voltage should be at as low a level as possible. Therefore, composite materials used for this purpose have to meet additional major requirements.
Efficient materials for this range include W/Cu, WC/Ag and WC/Cu in pure form or with further additives. In particular the matrix component Ag accounts for good current interrupting properties while the high-melting component W or WC minimizes the material consumption under the influence of an electric arc.
Due to the contradictory properties of the pure refractory components W and Cr, it is difficult to design contact materials for the remaining gap between 1,000 and 12,000 V which are able to meet the ever increasing requirements on switch boxes for vacuum contactors:
At an increasing voltage, the use of a pure tungsten component is limited by the increased tendency to emit electrons. This is due to the refractory nature of tungsten (melting point 3,410xc2x0 C.). Thus, the electric strength in the vacuum is weakened.
At a low voltage on the other hand, the use of a pure Cr refractory component is limited by the poor resistance to material consumption which is due to the accumulated material consumption resulting from the large number of operations.
It would now be conceivable to synthetically adjust a refractory component which would have an optimal profile regarding the melting point (i.e. the resistance to material consumption with respect to the switching property) and the electron emission (i.e. electric strength) depending on the desired voltage range by mixing the two differently melting metal portions Cr and W. Thus, during the switching of electrical contacts, it should be possible to neutralize the negative properties of the contact materials used so far (in the case of Cr the high degree of material consumption due to the low melting point, in the case of W the high degree of electron emission or low electric strength due to the high melting point).
One possible approach of this kind is described in EP-A-0 083 245 which discloses, inter alia, a CuCrW alloy which is prepared in a manner known per se involving powder metallurgy by compressing the metal powder mixture and sintering in a solid or liquid Cu phase. It is the object of this reference to prepare as finely-grained a composite as possible. This object is to be achieved by creating a completely solid solution of the refractory metals in each other due to the metals W and Cr which crystallize in a cubic system.
In order to examine the usefulness of this teaching, CuCrW composites were prepared in accordance with the instructions in this reference. After sintering in a liquid Cu phase, the W grains are present in their original form and size coated by Cr. The Wxe2x80x94Cr particles are embedded in the Cu matrix (FIG. 1). No mixed crystal of W and Cr of the type postulated in the reference could be detected. This is not surprising from a metallographic point of view since at the melting temperature of 1,100 to 1,200xc2x0 C. (above the Cu liquidus) which has to be applied for that process, no reaction of tungsten and chromium could be expected.
X-ray fluorescence analysis of the material Cu 71% /Cr 24% /W 5% prepared according to the reference showed up until the limit of detection that the tungsten is not soluble in the ambient matrix of Cr and Cu. The summation analysis over a Cr area of 10xc3x9714 xcexcm2 shows pure Cr, i.e. W is present in an amount below the limit of detection of  less than 0,1% (FIG. 2). On the other hand, a diffusion of Cr into the W particles could not be verified, either: a local analysis shows pure W, i.e. Cr is present in an amount below the limit of detection of  less than 0,1% (FIG. 3). Therefore, it seems as if a mutual penetration of the refractory metals Cr and W, i.e. the formation of a mixed crystal, cannot be realized. This reference does not provide a teaching to obtain improved switching properties by intimately mixing the refractory components Cr and W while taking advantage of the different properties of the two pure components.
Similarly, reference EP-A-0 668 599 discloses a contact material of CuCr with an additional auxiliary agent selected from the group consisting of tungsten, molybdenum, tantalum and niobium, which is obtained via diffusion of the refractory components in liquid copper phase and subsequent quenching as a fine-grained dispersion of the arc-resistant components in the Cu matrix.
With respect to a CuCrW material, a mutual diffusion of CrW as well as an arc-resistant grain of Cr and W is described. The invention is essentially directed to a fine-grained dispersion of the individual refractory components in the metal matrix. The formation of mixed crystals or intermetallic phases of the refractory constituents among each other is not described.
Thus, the requirements on materials for vacuum contactors to be used in the voltage range between 1,000 and 12,000 V are not met by the described mixtures of Cr and W in a Cu matrix.
It is therefore an object of the present invention to provide a composite material comprising a low melting, conductive matrix for example made from Cu or Ag and a granular additive of refractory components, which meets the mentioned requirements on vacuum power switches and vacuum contactors, i.e. which has both a high electric strength and thus a low electron emission and a high resistance to material consumption, and is therefore suitable for use in the voltage range of 1,000 to 12,000 V.
Another object of the invention is to provide a method for the production of such composite materials which can be carried out in an economically viable manner.
Finally, it is an object of the present invention to provide a composition material for the use as contact material, preferably as switching contact in electrical vacuum switch boxes in the voltage range of 1,000 to 12,000 V.
These objects were achieved based on the surprising finding that a material with advantageous properties is obtained if the refractory portion no longer consists of particles of one or more refractory components but if mixed crystals or intermetallic phases of at least two refractory components are present, wherein in a preferred embodiment these components have clearly different melting points. From a metallographic point of view the formation of an xcex1-phase consisting of the pure or highly concentrated refractory component cannot always be avoided at certain weight ratios. It is, however, decisive that mixed crystals or intermetallic phases of the used refractory components are formed as well, which leads to clearly improved properties (e.g. low electron emission) of the composite material. Preferably, compositions should be selected which exclude the formation of xcex1-phases.
According to the present invention, the desired material properties are not adjusted by sintering together, blending in additional components in the low-melting matrix or by mixing various high-melting powder components as is common in the prior art, but they are modified by means of prealloyed refractory components (present in the form of mixed crystals or intermetallic phases).
The present invention therefore relates to a powder-metallurgically produced composite material comprising a matrix from a metal with a melting point of at most 1,200xc2x0 C. and a granular additive embedded in this matrix consisting of at least two refractory components, characterized in that the refractory components are present in the form of mixed crystals or intermetallic phases.
Preferred embodiments of the composite material of the present invention have been made the subject matter of the claims. Especially preferred is a composite material wherein one or a first group of refractory component(s) having a melting point in the range of 1,500 to 2,400xc2x0 C. and the second or the second group of refractory component(s) having a melting point above 2,400xc2x0 C.
Furthermore, a method for producing said composite material is provided characterized in that a pulverized mixture of at least two refractory components is converted into a mixed crystal or an intermetallic phase by heating and then the powder obtained by cooling and pulverizing is combined with a metal matrix having a melting point of at most 1,200xc2x0 C. by means of a powder-metallurgical process.
Preferred embodiments of the process constitute the subject-matter of the claims.
A further subject-matter of the invention is the use of the mentioned composition material as electrical contact material, preferably as switching contact in electrical vacuum switch boxes, in particular in the voltage range of 1,000 to 12,000 V.